To prevent counterfeiting and to enable authenticity to be checked, security documents are typically provided with one or more security devices which are difficult or impossible to replicate accurately with commonly available means such as photocopiers, scanners or commercial printers.
One well known type of security device is one which uses a colour shifting element to produce an optically variable effect that is difficult to counterfeit. Such a colour shifting element generates a coloured appearance which changes dependent on the viewing angle. Examples of known colour shifting structures include photonic crystals, liquid crystals, interference pigments, pearlescent pigments, structured interference materials or thin film interference structures including Bragg stacks.
It is also known in the art that the optical effect produced by a colour shifting element can be modified by introducing a film comprising a surface relief over the colour shifting element, wherein the surface relief comprises a plurality of angled facets that refract the light incident to, and reflected from, the colour shifting element so as to provide a different optical effect to the viewer. For example, such an additional “light control” layer may produce colour shifting effects which are visible closer to a normal angle of viewing with respect to the device, and may enable more colours to be viewed on tilting the device as compared to the colour shifting element in isolation.
In order to increase the difficulty of counterfeiting such a security device, it is beneficial for the security device to exhibit more than one colour shifting effect. The amount of refraction of light (and therefore the exhibited colour shifting effect) is dependent at least upon the facet angles of the surface relief, and the use of different facet angles allows for different amounts of refraction and, correspondingly, different colour shifting effects. However, although this is beneficial for security, it is difficult to produce a surface relief having a plurality of different facet angles.